A Huge Unnamed Dwarf Planet, Almost the Size of Pluto, Has Been Hiding in Our Solar System

In a universe full of planets, 2007 OR10 is something special. It’s big, just slightly smaller than the size of Pluto. And it’s close, within our very own solar system. So how did it still manage to take astronomers by surprise?

Researchers from Konkoly Observatory just revealed in the Astronomical Journal that they had uncovered new details about planet 2007 OR10 that show it to be the third largest dwarf planet ever seen in our solar system. New planets are uncovered all the time, thanks to Kepler. In fact, the mission just unearthed a stash of more than 1,200 new exoplanets, bringing its total haul to over 3,200. 2007 OR10 has a diameter of 955 miles, which makes it a relative monster. The only larger dwarf planets in our solar system are Pluto, which has a diameter of 1475 miles, and Eris, which has a diameter of 1445 miles. 2007 OR10 is the third biggest dwarf planet in our solar system—and the largest unnamed planet of any kind within our solar system.Image: Konkoly Observatory/András Pál, Hungarian Astronomical Association/Iván Éder, NASA/JHUAPL/SwRI

Even dwarf planets that are considerably smaller, like Haumea and Makemake, are considered big enough to need names.

Researchers have known a planet was out there since 2007, but they vastly underestimated just how big it is. The reason its tremendous size wasn’t determined until recently lies in both the planet’s dark surface and its strange orbit. The surface of the planet is an incredibly dark red, perhaps due to a ever-changing covering of methane ice. That lack of reflective light made it hard for Kepler to even spot the planet, much less know its true size. The planet also has an incredibly slow rotation time that gives it a 45 hour day, one of the solar system’s longest. That slow rotation, plus a long, elliptical orbit, made it hard to spot the planet for long—although Kepler managed to catch brief glimpse in 2014, as you see here:

The dwarf planet could have easily continued to evade astronomers examination if the Konkoly researchers hadn’t thought to pair NASA Kepler data with ESA Herschel data. By combining information about the amount of light the planet was reflecting from Kepler with information about its heat radiation from Herschel, researchers were finally able to calculate an accurate size measurement. The finding also suggests an avenue for learning much more about all those little (or perhaps not-so-little) new planetary discoveries that are popping up all the time now.

For 2007 OR10, these results mean both that we’re finally aware of the existence of one of the largest dwarf planets nearby and that we know more about what that planet is like. With the new size measurements confirmed, the Palomar Observatory astronomers who discovered the planet back in 2007 have also already begun contemplating possible names. From there, the dwarf planet will begin the process of being recognised by the International Astronomical Union, which could take a while to finally wrap up. Still, it means that 2007 OR10 won’t be keeping that designation for long.